The present invention relates to a method for fabricating thin-film image sensing devices and, more particularly, to a method for fabricating image sensing devices that include electrodes of transparent conducting material connected to thin-film metal conductors.
In the development of high-speed, high-resolution facsimile equipment, it has become necessary to provide image sensors comprising large, high density arrays of photodetectors which are capable of fast response and high sensitivity to small changes in light intensity. A typical scanning arrangement used in facsimile equipment is illustrated in FIG. 1. With reference with FIG. 1, a manuscript 1 moves in a transverse direction relative to a linear image sensor 2 comprising a linear array of regularly spaced photodetectors 3, which extend across the width of the manuscript 1. The image of one line of the manuscript at a time is focused onto the photodetector array 3 by a Selfoclens 5. The manuscript is illuminated by two linear arrays of light emitting diodes 4 situated on each side of the Selfoclens 5.
In order to meet the requirements of high speed, high-resolution facsimile equipment currently being developed, it is generally desirable for the photodetectors of the linear array to have a pitch of approximately 8 per mm or 16 per mm, depending upon the expected size of the patterns in the manuscript to be scanned. The active area of each photodetector should be approximately 100 .mu.m.times.100 .mu.m for the larger pitch and 50 .mu.m.times.50 .mu.m for the smaller pitch. Furthermore, the response time of the photodetectors should be such that an A4 or B4 size manuscript can be scanned at the rate of one line in 4 msec or less.
Formerly, image sensors for facsimile equipment have been constructed with linear arrays of charge-coupled devices (CCD's); however, recently there has been developed image sensors constructed with linear arrays of thin-film semiconductor p-i-n photodiodes. The latter construction provides the advantages of allowing larger arrays to be fabricated with improved performance and lower manufacturing cost.
An exemplary structure for a thin-film, p-i-n photodiode array image sensor is illustrated in FIGS. 2 and 3. Referring to FIGS. 2 and 3, the image sensor 20 comprises a plurality of regularly spaced p-i-n photodiodes 51 disposed in a row. The photodiodes 51 are fabricated by first forming a plurality of separate, regularly-spaced, transparent conducting layers 21 on a major surface of a glass substrate 10. Each of the transparent conducting layers 21 has a square electrode region 22 having dimensions of approximately 100 .mu.m.times.100 .mu.m or 50 .mu.m.times.50 .mu.m, depending upon the pitch of the photodiode array, and a strip-like connecting region 23 extending from one side of the electrode region 22. The layers 21 are formed by depositing a film of either indium tin oxide or SnO.sub.2 to a thickness in the range of 500.ANG. 2000.ANG. by conventional electron beam evaporation, sputtering or chemical vapor deposition techniques. Following deposition, the transparent conducting film is patterned to form the separate layers 21 by conventional photolithography and etching techniques.
The image sensor 20 further includes an amorphous silicon layer 31 of approximately 1 .mu.m in thickness formed by conventional glow discharge decomposition of SiH.sub.4 gas at a relatively low temperature in a reaction chamber containing the substrate 10. The amorphous silicon layer 31 is formed over a predetermined area of the substrate 10, which includes the electrode portion 22 of the transparent conducting layers 21, by using an appropriate metal mask over the substrate 10 while the layer 31 is being formed. During deposition of the amorphous silicon layer 31, diborane gas is initially introduced into the reaction chamber for an appropriate time to create a boron doped p-type impurity layer 32 of approximately 100.ANG. in thickness adjacent the bottom surface of the layer 31, and phosphine gas is introduced into the reaction chamber for an appropriate time at the end of the deposition to create a phosphorus doped n-type impurity layer 33 of approximately 500.ANG. in thickness at the top surface of the layer 31. The portion of the amorphous silicon layer 31 between the p-type and n-type layers 32 and 33 is undoped (i.e., intrinsic).
Following the deposition of the amorphous silicon film, a layer of aluminum 41 of approximately 1 .mu.m in thickness is deposited over a predetermined area of the substrate 10, such that the aluminum layer 41 overlies the electrode portions 22 of the transparent conducting layers 21 and is separated from such layers 21 by the amorphous silicon layer 31. The aluminum layer 41 is formed by conventional electron beam evaporation, while the substrate surface is covered by an appropriate mask.
The above-described image sensor 20 consists of an array of p-i-n photodiodes 51 having a common aluminum cathode electrode 41 and individual transparent anode electrodes 22. The photodiodes 51 are responsive to light incident thereon from the underside of the glass substrate 10 through the transparent anode electrodes 22. The photodetection signals produced by the photodiodes 51 are respectively provided through the connecting regions 23 of the transparent conducting layers 21. The photodetection signals are coupled to an integrated circuit 50 mounted on the substrate 10, which reads and processes such signals. Since the transparent conducting layers 21 have relatively high sheet resistances and the photodiode array 20 is separated from the integrated circuit 50 by a relatively large distance, the conductors 42 that couple the photodetection signals to the integrated circuit 50 are made of low sheet resistance thinfilm metal strips, in order to avoid excessive attenuation of the signals being coupled. The metal strip conductors 42, which are formed by photolithography and etching, contact the connecting regions 23 of respective transparent conducting layers 21 at one end and are connected to respective bonding pads 51 of the integrated circuit 50 at the other end by bonding wires 60 attached by conventional stitch bonding.
For reasons of optimizing the manufacturing yield of the image sensor 20, it is desirable to form the metal strip conductors 42 in a separate metallization step than the one used to form the common cathode electrode 41 and to complete the patterning of the metal strip conductors 42 before the deposition of the amorphous silicon layer 31.
Turning now to FIGS. 4 and 5, there is illustrated a known technique for patterning the metal strip conductors 42 by photolithography and etching. Initially, the transparent conducting layers 21 are formed on the a major surface of the substrate 10 in the manner described above. After patterning of the transparent conducting layers 21, a first metal film 40 consisting of aluminium approximately 1 .mu.m in thickness is deposited over the entire substrate surface by electron beam evaporation followed by the application, exposure and development of a photoresist layer 70 to form an etch mask for the first metal film 40. The pattern of the photoresist layer 70 after development is the desired pattern of the metal strip conductors 42 shown in FIG. 2. Following development, the photoresist layer 70 is subjected to a post-development bake (post bake) at a temperature of approximately 120.degree. C., and the aluminium layer 40 is then etched in a 1:1 mixture of phosphoric acid and nitric acid to remove those portions of the first metal film 40 that are not covered by the patterned photoresist layer 70. After etching the photoresist layer 70 is stripped off to expose the metal strip conductors. The above-described known technique for forming the metal strip conductors has the problem in that the material of the transparent conducting layers 21, which is otherwise impervious to the etchant for aluminum, is reduced to its metallic components by hydrogen gas released when the aluminum film 40 is being etched. Since such metallic components are attacked by the etchant, the known technique for forming the metal strip conductors results in undesirable erosion or total removal of the transparent conducting layers 21.
Accordingly, a need exists for a method for fabricating thin-film photodiode array image sensing devices of the above-specified type in which the metal strip conductors for coupling photodetection signals to signal processing circuitry are formed without erosion or removal of the transparent conducting layers that form the anode electrodes of the photodiodes.